1. Field of the Invention
The present invention is directed to a process of debinding ceramic products, and more particularly to a process of removing an organic binder contained in the ceramic products by heating the same within a furnace at the presence of a surrounding gas.
2. Description of the Prior Art
In general, ceramics products are formed by baking a mixture of ceramic powders and an organic binder which is added to impart plasticity or fluidity to the mixture for facilitating the ceramic products in press, extrusion, injection molding, and various other forming method. In particular, the injection molding is mostly preferred to form ceramic products into complicated shapes. The injection molding requires high fluidity and therefore more amount of the organic binder to be mixed with the ceramic powders. However, the excess amount of organic binder will impair the finished ceramic product and should be therefore removed after the forming of the ceramic product. Although the ceramic product can be removed of the organic binder by sintering at which the organic binder is vaporized to be flown outwardly of the ceramic product. However, the heat-treatment should be carefully controlled in order to avoid cracking, flaking, or fracture of the ceramic product. In fact, when the ceramic product is heated too excessively or abruptly, the organic binder within the produce is rapidly decomposed to generate the cracked gas in an excess amount per unit time to thereby increase an internal gas pressure within the ceramic product to such an extent as to cause the swelling of the product, which eventually results in the cracking or flaking in the surface of the products or even in the fracture.
To avoid the occurrence of such defects during the heat-treatment, there have been conventionally proposed to control the temperature of the furnace in a feed back manner in accordance with a predetermined program by monitoring the furnace temperature. Such prior process is disclosed in Japanese patent early publication (KOKAI) No. 61-163172 in which a control is made to vary the furnace temperature in accordance with a monitored change in the weight of the ceramic product, and in Japanese early publication (KOKAI) No. 62-7674 in which a control is made to vary the furnace temperature based upon a monitored change in concentration of a particular component of the cracked gas.
However, because of the finding that the amount of the cracking gas of the organic binder is not directly related to the temperature, the above prior temperature control alone is not effective in preventing the generation of excess amount of the cracked gas and fail to achieve desired defect-free debinding of the product. In fact, it is found that the generation of the cracked gas can be well controlled by varying the amount of the surrounding gas being supplied to the furnace per unit time, or flow rate of the surrounding gas, in addition to varying the furnace temperature. As seen in FIGS. 1A to 1G which schematically illustrate the process of debinding a ceramic product 20, when the surrounding gas is supplied in a proper amount or at a proper flow rate around the product 20, the cracked gas 30 emerging from the product 20 is diffused in the surrounding gas and carried thereon to be expelled out of the furnace, thereby allowing the cracked gas to continuously escape from the product 20 (shown in FIGS. 1A to 1D). On the other hand, when the surrounding gas is supplied in a less amount or at a less flow rate, it will be soon saturated with the cracked gas emerging from the product 20 which in turn retards the cracking of the organic binder in the surface layer 31 of the product 20 such that the organic binder becomes tarry or solidified to thereby inhibit the escape of the cracked gas or the organic binder from within the product 20 (FIGS. 1A to 1G). With this result, the internal gas pressure of the product is unduly increased to such an extent as to cause the fracture of the product 20 (FIG. 1G). Also, when the surrounding gas is supplied in an excessive amount, the cracked gas will be generated and drawn too rapidly from the outer surface layer of the product, leaving therein voids causing the unacceptable surface flaking.
Further, when air is utilized as the surrounding gas rather than an inert gas for economical reasons, an oxygen concentration of the surrounding air is also found to be an important factor in controlling the generation of the cracked gas during the heat-treatment process. This is because that, as schematically illustrated in FIG. 2A, as the oxygen concentration is increased, the organic binder has its inter-carbon bonds broken at a more number of sites by oxygen (indicated by dotted lines) to thereby generate the cracked gas of a correspondingly lower molecular weight which will occupy a larger volume within the ceramic product. With this result, the internal gas pressure of the cracked gas within the ceramic product is increased to make the product more susceptible to the above defect. When utilizing an inert gas such as nitrogen as the surrounding gas, the organic binder has a less number of breaks (indicated by dotted lines in FIG. 2B) in the inter-carbon bonds to thereby generate the resulting cracked gas of a high molecular weight and therefore occupying less volume within the product, contributing to well inhibiting the occurrence of the above defects. However, in view of the fact that the debinding process requires to continuously flow the surrounding gas around the ceramic product over a relative long period of time and therefore use a large volume of the surrounding gas, the use of the inert gas of relatively expensive nature poses a serious cost problem and therefore should be avoided or minimized.
The above discussion can be more clearly understood from FIG. 3 which is provided to illustrate a change of carbon monoxide [CO] concentration, one component of the cracked gas generated from the organic binder contained in the ceramic product during the process of heating the product in accordance with a predetermined time-temperature curve while supplying the surrounding air at a fixed rate. As apparent from the figure, during the process of controlling the temperature of the ceramic product in this condition of supplying the surrounding air at a fixed rate there occurs an abrupt increase of CO concentration, which is indicative of the development of the defects in the product, at a point not directly related to the temperature. Therefore, it is confirmed that only the temperature control fails to achieve the defect-free debinding of the ceramic product. In the figure, a curve of an oxygen concentration is also shown for reference.
Further, the feed back control of the furnace temperature alone is found to pose another problem that the furnace and the ceramic product is difficult to be heated exactly in an intended manner due to a relatively large heat capacity of the furnace and the ceramic product, and is most likely to suffer from an overshooting of temperature, which is also the cause of the defects in the ceramic product and may make the temperature control itself ineffective for achieving a defect-free debinding of the ceramic product.